The mass distribution of clumps within infrared dark clouds. A Large APEX Bolometer Camera study

Aims. We present an analysis of the dust continuum emission at 870 mu m in order to investigate the mass distribution of clumps within infrared dark clouds (IRDCs). Methods. We map six IRDCs with the Large APEX BOlometer CAmera (LABOCA) at APEX, reaching an rms noise level of sigma(rms) = 28 -44 mJy beam(-1). The dust continuum emission coming from these IRDCs was decomposed by using two automated algorithms, Gaussclumps and Clumpfind. Moreover, we carried out single-pointing observations of the N2H+ (3-2) line toward selected positions to obtain kinematic information. Results. The mapped IRDCs are located in the range of kinematic distances of 2.7-3.2 kpc. We identify 510 and 352 sources with Gaussclumps and Clumpfind, respectively, and estimate masses and other physical properties assuming a uniform dust temperature. The mass ranges are 6-2692 M-circle dot (Gaussclumps) and 7-4254 M-circle dot (Clumpfind), and the ranges in effective radius are similar to 0.10-0.74 pc (Gaussclumps) and 0.16-0.99 pc (Clumpfind). The mass distribution, independent of the decomposition method used, is fitted by a power law, dN/dM proportional to M-alpha, with an index (alpha) of -1.60 +/- 0.06, consistent with the CO mass distribution and other high-mass star-forming regions.